1. Field of the Invention
The present invention relates to a presbyopia correction contact lens whose optical portions respectively provide a near vision correction region and a distant vision correction region having a common optical center.
2. Discussion of the Related Art
As a contact lens for vision correction of an eye suffering from decreasing accommodation, such as presbyopia, there has been proposed a multifocal-type presbyopia correction contact lens having a plurality of vision correction regions which provide respectively determined mutually different optical powers. The multifocal-type presbyopia correction contact lens is generally classified into two types: an alternating or translating vision type contact lens, wherein the vision correction regions including a near and a distant vision correction region are selectively and alternatively used as needed in observing objects, with a shift of the visual axis of the lens user; and a simultaneous vision type contact lens, wherein the vision correction regions are simultaneously used, and a desired image observed through one of the vision correction regions is selected by an action of the lens wearer's brain. In the field of contact lenses, the simultaneous vision type contact lens prevails over the alternating vision type contact lens since it is rather difficult to select an appropriate vision correction region for observing the object at a desired distance.
As one example of the simultaneous vision type presbyopia correction contact lens, there is proposed a so-called bifocal contact lens having two focal points which respectively correspond to the near and distant vision correction regions.
U.S. Pat. No. 4,636,049 discloses one example of the bifocal contact lens, wherein the back surface of the optical zone of the lens, i.e., a cornea-contacting surface, serves as a base curved surface having a profile corresponding to that of the cornea of the lens user's eye, and two concentric regions having respective different radii of curvature are formed on the front surface of the optical zone, so that the two concentric regions provide two optical portions which serve as one and the other of the near and distant vision correction regions.
The bifocal contact lens constructed as described above, however, tends to suffer from various problems such as ghosting or double imaging, and mutual interference of the images obtained through the near and distant vision correction regions, since a bend on which a point of change of the curvature lies is present at a boundary of the two optical portions, and the optical power abruptly changes between the values of the near and distant vision correction regions at the boundary. In the bifocal contact lens disclosed in the above publication, the bend or discontinuity at the boundary of the two optical portions is present on the front surface of the contact lens. Accordingly, the contact lens undesirably suffers from a flare resulting from a large difference between the refractive indices of the contact lens and the air, thereby causing scattering of light.
In the bifocal contact lens disclosed in the above publication, the near vision correction power and distant vision correction power are provided by forming, on the front surface of the contact lens, the two optical portions having different respective curvatures. When the contact lens is formed by a molding operation using a mold assembly consisting of a female mold and a male mold which respectively give the front surface and the back surface of the contact lens, it is necessary to prepare a large number of different female molds giving different front surfaces of the contact lens by polymerization of a lens material in the mold assembly, depending upon different combinations of the near and distant vision correction powers to be given to different contact lenses. This undesirably pushes up the cost of manufacture of the contact lens.
JP-A-57-105717 discloses another example of the bifocal contact lens, wherein two concentric regions having different radii of curvature are formed on the back surface of the optical zone of the contact lens, so that the distant vision correction region is provided by one of the two concentric regions which corresponds to a central optical portion, while the near vision correction region is provided by the other concentric region which corresponds to a peripheral optical portion. The thus constructed contact lens also suffers from the ghosting or double imaging problem in observing the objects, arising from the bend or discontinuity at the boundary of the two optical portions.
In the contact lens disclosed in the above publication, the radius of curvature of the back surface of the central optical portion is made considerably smaller than that of the back surface of the peripheral optical portion. Due to a large difference between the radii of curvature of the respective back surfaces of the two optical portions, a large amount of tear fluid tends to be accumulated between the back surface of the central optical portion of the contact lens and the corneal surface of the lens user's eye. In this case, the optical power of the contact lens during its use on the lens user's eye tends to fluctuate due to refraction of light caused by the tear fluid. Accordingly, the contact lens does not provide the intended optical power. Further, the cornea of the lens user's eye undesirably deforms so as to follow the configuration of the back surface of the central optical portion of the contact lens while the contact lens is worn on the eye, deteriorating the wearing comfort of the contact lens and causing cornea trouble.